Here is my understand: Digital sensors can record all light wave band widths. Filters are placed over sensors to remove those bands not visible to the human eye. Otherwise the image would represent what you saw when shooting.

To convert a camera the IR filter must be removed, and the camera becomes a dedicated IR camera. I believe the same is true with UV too.

Is it really necessary to convert? And really what does that do to the camera?

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No, it is not necessary to convert, but unless you do so, you will need much longer exposures to capture the IR. I don't have a time comparison to offer you, but someone here may. If you do convert, it Wil restrict that camera to IR since it will now have a permanently affixed IR filter.

Without the 'hot mirror' (the built in IR/UV filter) CMOS & CCD sensors as used in digital cameras are relatively sensitive to both UV & IR but only within limited wavelengths. The actual response to any wavelength will vary depending on the sensor design. Likewise the choise of hot mirror will effect how sensitive an unmodified camera is.
Neither sensor will respond to mid infrared (2500-50000nm) let alone far infrared as needed for thermal imaging. I don't think either respond much below 300nm in the UV (below ~200nm air absorbs UV).
To further complicate things the lenses used will vary considerably in their transmission of the non visible radiation - special quartz lenses are sold for UV work and many quality optical lenses produce hot spots when used for IR.

As I mentioned in another of the IR threads, yesterday I had a bit on sun at lunchtime (seems like only the third time I 've seen it since september) so had a quick play with my unmodified G1 with IR. Using a 720nm filter the exposure was about 2000 longer than with visible, this is very roughly similar to my K100d - just about handholdable at moderate ISO. When I tried with my Pentax K7 I think the exposures needed to be at least 200 times longer still, certainly no option but a tripod & long exposures even at its highest ISO (~1s at ISO 6400 wide open).
[update] I've just tried editing the IR images from the G1, & I'm not convinced they're showing proper IR characteristics. Simply desaturating has not produced the pale foliage I'm used to. It could be just leakage of visible light, or perhaps its just showing the extreme edge of the IR..

It is possible to have a camera converted to 'full spectrum' making it sensitive to UV, visible & IR. If you do this you can use filters in front of the lens to select which you want to use - a bit of a pain but the ultimate in flexibility.

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